As the integration density and complexity of integrated circuits continues to increase, it may be increasingly difficult to properly align the patterned layers of an integrated circuit to one another. As is well known to those having skill in the art, integrated circuits are formed on an integrated circuit substrate, such as a semiconductor substrate, by sequentially patterning layers on the integrated circuit substrate. The registration accuracy of the layers to the substrate may be a limiting factor in the integration density of the integrated circuit and the number of layers that can be included in the integrated circuit. Accordingly, integrated circuit exposure tools including but not limited to proximity printers, projection printers, aligners and steppers, generally provide systems, methods and computer program products for aligning a pattern that is being projected to underlying patterns and/or to the underlying substrate.
Generally, alignment of one patterned layer to underlying layers may be performed with the assistance of special alignment marks that are designed into each mask level. When the alignment marks are aligned, it may be assumed that the remainder of the patterned layer is also properly aligned. Adjustment of the alignment marks was originally performed by human operators using a microscope. However, decreasing feature sizes and the increasing number of alignments per wafer have contributed to the development of automatic alignment for exposure tools. See, for example, U.S. Pat. No. 5,734,594 to Chu et al., entitled "Method and System for Enhancement of Wafer Alignment Accuracy"; U.S. Pat. No. 5,684,595 to Kato et al. entitled "Alignment Apparatus and Exposure Apparatus Equipped Therewith"; U.S. Pat. No. 5,682,243 to Nishi entitled "Method of Aligning a Substrate "; U.S. Pat. No. 5,680,200 to Sugaya et al. entitled "Inspection Apparatus and Method for Optical System, Exposure Apparatus Provided With the Inspection Apparatus, and Alignment Apparatus and Optical System Thereof Applicable to the Exposure Apparatus"; U.S. Pat. No. 5,674,650 to Dirksen et al. entitled "Method of Repetitively Imaging a Mask Pattern on a Substrate, and Apparatus for Performing the Method"; and U.S. Pat. No. 5,657,129 to Nishi entitled "Method and Apparatus for Alignment of a Substrate".
In conventional automated alignment, an alignment mark for a layer is imaged, for example by scanning the alignment mark or by using a camera to image the alignment mark. The imaged alignment mark may be used to produce an alignment signal. The center of the alignment signal is then determined and this center position is used to align the alignment mark to an underlying alignment mark, or to a reference position in the exposure tool's coordinate system.
Unfortunately, the alignment mark may not be symmetrical, thereby making it more difficult to find the center position of the alignment mark. Moreover, even if the alignment mark is symmetrical, subsequent processing which places a coating including one or more layers on the alignment mark and/or misalignment, aberration or uneven illumination of the alignment subsystem optics, may produce an asymmetrical alignment signal.
For example, the alignment mark may be a mesa or a trench on an integrated circuit substrate. One or more layers may form a coating on the substrate. The layers may be formed asymmetrically on the alignment mark, due to the topography of the trench or mesa and/or due to asymmetries in the coating forming process. In one specific example, Chemical Mechanical Polishing (CMP) is widely used to planarize a coating. The CMP process may give rise to asymmetry in the coating. Other processes, such as metal deposition and photoresist coating may also produce asymmetries. When detecting the alignment mark by imaging through the coating, these asymmetries may make it difficult to accurately define the center of the asymmetric alignment signal.
One technique for detecting the center of an alignment signal is described in U.S. Pat. No. 4,886,974 to Ina entitled "Mark Detecting Device for Detecting the Center of a Mark by Detecting Its Edges". In this patent, a device detects the center of an alignment mark in accordance with a photoelectrically-obtained signal, while taking into account the degree of asymmetry of the signal obtained at the time of optical scanning of the mark. First and second signals related to first and second edges of the alignment mark are detected. The detected first and second signals are added to produce a resultant signal, and the resultant signal is processed in accordance with any difference between the first and second signals, to determine the position of the alignment mark.
However, notwithstanding the above-described techniques, there continues to exist a need for improved systems, methods and computer program products for detecting the position of an alignment mark, notwithstanding asymmetries in the alignment mark, fabrication process-induced asymmetries in a coating on the alignment mark, and/or misalignment, aberration or uneven illumination of the alignment subsystem optics.